1. Field of the Invention
The invention relates to a control apparatus and a control method for an internal combustion engine and, more particularly, to a control apparatus and a control method that are capable of restraining the deterioration of exhaust gases of a multicylinder internal combustion engine when the air/fuel ratio varies among the cylinders of the engine.
2. Description of Related Art
Generally, in an internal combustion engine equipped with an exhaust . gas control system that uses a catalyst, it is essential to control a mixing ratio of air and fuel in a mixture that is burned in the internal combustion engine, that is, the air/fuel ratio, in order to accomplish high-efficient removal of pollutants from exhaust gas through the use of the catalyst. In order to perform such a control of the air/fuel ratio, an air/fuel ratio sensor is provided in an exhaust passageway of the internal combustion engine, and a feedback control is carried out so as to cause the air/fuel ratio detected by the sensor to be equal to a predetermined target air/fuel ratio.
Usually, in a multicylinder internal combustion engine, the air/fuel ratio control is performed by using the same control amount for all the cylinders; therefore, despite the execution of the air/fuel feedback ratio control, the actual air/fuel ratio can vary among the cylinders. In such a case, if the variation of the air/fuel ratio is of a small degree, the variation of the air/fuel ratio can be absorbed by the air/fuel ratio feedback control and pollutants in exhaust gas can be removed by the catalyst. Thus, small degrees of variation of the air/fuel ratio do not affect the exhaust gases, and thus do not lead to any particular problem.
However, if the air/fuel ratio greatly varies among the cylinders due to, for example, failure of the fuel injection systems of one or more cylinders or the valve actuation mechanism of the intake valves, etc., the exhaust gas deteriorates, and problems arise. Such a large variation in the air/fuel ratio as to deteriorate the exhaust gases is detected as an abnormality.
Japanese Patent Application Publication No. 2009-74388 (JP 2009-74388 A) discloses an example of the abnormality detection mentioned above. In an internal combustion engine described in Japanese Patent Application Publication No. 2009-74388 (JP 2009-74388 A), a main air/fuel ratio feedback control in which the air/fuel ratio detected by using a pre-catalyst sensor that is provided in an exhaust passageway on an upstream side of a three-way catalyst is caused to be equal to a stoichiometric ratio, and an auxiliary air/fuel ratio feedback control in which the air/fuel ratio detected by using a post-catalyst sensor provided at the downstream side of the three-way catalyst is caused to be equal to the stoichiometric ratio are executed. Furthermore, as a result of comparison between two cases in which the amount of deviation of the total air/fuel ratio is the same, it has been shown that the amount of hydrogen in exhaust gas discharged into an exhaust passageway tends to be larger when the air/fuel ratio of a cylinder alone is greatly deviated to the rich side than when the air/fuel ratios of all the cylinders are small, and are equally deviated to the rich side. Since such hydrogen is able to be removed by the three-way catalyst, there occurs a difference between the air/fuel ratio detected by the pre-catalyst sensor and the air/fuel ratio detected by the post-catalyst sensor when there is abnormality of air/fuel ratio variation among the cylinders. On the basis of this relation, the variation abnormality in the air/fuel ratio among the cylinders is detected in the internal combustion engine of the foregoing JP 2009-74388 A.
In order to appropriately perform the removal of pollutants in exhaust gas through the use of the catalyst as described above, it is permissible to execute such a feedback control that the exhaust air/fuel ratio is caused to be equal to a predetermined target air/fuel ratio. However, when the variation abnormality in the air/fuel ratio among the cylinders is present, it is not easy to appropriately control the air/fuel ratio by the aforementioned air/fuel ratio feedback control.